1. Field
The present invention relates to an optical apparatus that is suitable for a passive optical network (PON) system and may be applied to a control of power of upward signal lights from a plurality of optical network units (ONU) to an optical line terminal (OLT).
2. Description of the Related Art
A passive optical network (PON) system is known as an optical fiber network system suitable for ordinary households' subscribers. FIG. 7 is a diagram illustrating a configuration of a PON system. In the PON system of FIG. 7, a signal light is transmitted to an optical transmission line 130 from an optical line terminal (OLT) 110, which is provided in a central station, and the signal light is branched into signal lights by an optical coupler 140. Optical network units (ONU) 120-1, 120-2, and 120-3, which are installed in subscriber's homes, are connected to the optical coupler 140 via lines.
The OLT 110 is an apparatus that transmits a downward signal light and distributes information to each of the ONUs 120. The OLT 110 also receives upward signal lights from the respective ONUs 120. Each of the ONU 120 is an apparatus that performs communication control, such as communication with the OLT 110, and conversion between a signal light and an electric signal. Here, the respective upward and downward signal light are bidirectionally transmitted using different wavelength via the one optical transmission line 130, between the OLT 110 and the optical coupler 140.
The downward signal light from the OLT 110 to each of the ONUs 120 is transmitted using time division multiplexing (TDM) as a signal light in a continuous mode. The ONU 120 detects frame synchronization information and management information that are included in the downward signal light. On the basis of the frame synchronization information and management information, the ONU 120 retrieves data in one of the time slots that are assigned to the respective ONUs 120. The time slot corresponding to the respective ONU 120 is assigned in advance.
The upward signal lights transmitted from the respective ONUs 120 to the OLT 110 are transmitted as signal lights in a burst mode in accordance with predetermined timings. The timings are provided from the OLT 110 to the respective ONUs 120 so that the upward signal lights do not collide with one another.
FIG. 8 is a diagram illustrating a configuration of a PON system in which a relay amplifier is disposed. As illustrated in FIG. 8, in order to transfer a signal light over a longer distance, a relay amplifier 150 is provided on the optical transmission line 130 between the OLT 110 and the optical coupler 140. In the relay amplifier 150, upward and downward signal light, which are bidirectionally transmitted, are separated from each other by WDM couplers 151 and 152. Loss of the upward and downward signal lights caused by optical transmission losses are compensated by optical amplifiers 153 and 154, respectively.
FIG. 9 is a diagram illustrating a configuration of an OLT in which a pre-amplifier for upward signal light is disposed. As illustrated in FIG. 9, in the OLT 110, a transmitter 111 outputs a downward signal light with a higher power. Upward signal lights from a WDM coupler 113 is amplified by a pre-amplifier 114 and optical transmission losses are compensated. After unnecessary noise light is removed by an optical filter 115 in upward signal lights, a receiver 112 receives the upward signal lights. Configurations illustrated in FIG. 9 are discussed in “Technology Search: Future Prospects for Optical Network”, Kuniaki Motoshima, Journal of the Institute of Electrical Engineers, 2006, Volume 126 Number 2, P. 92-96), for example.
In the above-described PON system that supports a longer distance, illustrated in FIG. 8 or 9, with respect to upward signal lights, while levels of the upward signal lights as a whole are compensated by the relay amplifier 150 or the pre-amplifier 114 in the OLT 110, a deviation ΔP of optical power values P1, P2, P3, . . . of signal lights corresponding to the time slots, is not compensated and remains. The deviation ΔP is caused by a variation in losses that occur between the respective ONUs 120 and the optical coupler 140, as illustrated in FIGS. 8 and 9.
In a PON system supporting high-speed optical communication, for example, in which a bit rate of signal light is equal to or higher than 10 Gbps, as a dynamic range of a receiver in an OLT is narrow, the deviation ΔP of optical power values of upward signal lights degrades reception characteristics of the OLT.